Comparison of microwave dielectric behavior
between Bi
1.5
Zn
0.92
Nb
1.5
O
6.92
and Bi
1.5
ZnNb
1.5
O
7
M.-C. Wu
a, S. Kamba
b, V. Bovtun
b, W.-F. Su
a,∗aDepartment of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan bDepartment of Dielectrics, Institute of Physics, ASCR, Praha, Czech Republic
Available online 8 November 2005
Abstract
The microwave dielectric, Bi1.5ZnNb1.5O7 exhibits low-temperature dielectric relaxation. To find the origin of the dielectric relaxation of
Bi1.5ZnNb1.5O7, we studied the structure and dielectric behavior of Bi1.5ZnNb1.5O7 in detail. The Bi1.5ZnNb1.5O7 is not composed of a single
phase pyrochlore structure. Instead, it consists of unusual structure of Bi1.5Zn0.92Nb1.5O6.92and ZnO. The ZnO is distributed evenly in the grain
and at the boundary of the Bi1.5Zn0.92Nb1.5O6.92structure. Many small voids (<1m) were observed in the samples due to the loss of volatile Bi
during sintering. The Bi1.5Zn0.92Nb1.5O6.92exhibited a broad dielectric relaxation between 100 and 400 K at 1.8 GHz, peaking around 230 K. The
Fourier transformation IR spectra predict that dielectric relaxation may occur near room temperature during extremely high frequencies (THz). The substitutional point defects in Bi1.5Zn0.92Nb1.5O6.92provide room for dielectric relaxation at microwave frequencies. The low quality factor
Q× f (∼520 GHz) of Bi1.5Zn0.92Nb1.5O6.92results from both the dielectric relaxation of the material and the voids within its microstructure. The
presence of ZnO phase in the Bi1.5ZnNb1.5O7produces interstitial defects that further enhance the dielectric relaxation with reduced quality factor
Q× f (∼426 GHz).
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Powders-solid state reaction; Dielectric properties; Microstructure
1. Introduction
The demands for miniaturization in microwave communi-cation technologies require continual discovery and innovation in the development of new materials. Most conventional ceramics have excellent microwave dielectric properties, such as BMT (BaMg1/3Ta2/3O3), BNT (BaO-Nd2O3-TiO2),
etc., and sinterabilities above 1300◦C. Considering the high sintering temperature, Ag–Pd electrode is the only choice for multilayer ceramic components (MLCCs). In the microwave frequency range, the dielectric loss of components is mostly attributed to the electrode. Good conductivity of the electrode is important for MLCCs. Thus, it is desirable to replace the poorly conducting and high-cost Ag–Pd electrode with silver electrodes which have better properties and lower cost. How-ever, since the melting temperature of silver is low (961◦C), a low sintering temperature material is required to cofire with silver.
∗Corresponding author. Tel.: +866 2 3366 4078; fax: +866 2 3366 4078. E-mail address: suwf@ntu.edu.tw (W.-F. Su).
Recently, Bi1.5ZnNb1.5O7(BZN) has emerged as a good
low-sintering (∼1000◦C) microwave material because it exhibits
high dielectric constant and low-temperature coefficient of resonance frequency (τf).1–7 Its sintering temperature can be
decreased to 950◦C by adding 3.0 wt% BaCO3–CuO into BZN8
and even lower to 850◦C by adding 2 mol% V2O5 to BZN.9
However, BZN exhibits low temperature (0–200 K) dielectric relaxation behavior at 1 MHz. Upon heating, the relaxation fre-quency steeply increases according to the Arrhenius law and appears in microwave range at room temperature.10,11
Levin et al.12 investigated the structure of Bi1.5ZnNb1.5O7
that is composed of an unusual cubic pyrochlore single phase consisting of Bi1.5Zn0.92Nb1.5O6.92and small amounts of ZnO.
In order to find the origin of the dielectric relaxation of Bi1.5ZnNb1.5O7, we studied the structures and dielectric
behav-iors of Bi1.5ZnNb1.5O7in detail. The results are compared with
that of Bi1.5Zn0.92Nb1.5O6.92.
2. Experimental
Reagent grade oxide powders with an appropriate molar ratio of Bi2O3, ZnO, and Nb2O5(99.9% purity each, Alfa Chemicals, 0955-2219/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
USA) were used to prepare BZN samples using the conventional mixed solid method. The samples were calcined at 900◦C for 2 h. The crystalline phases of the calcined BZN powder samples were comparable to those reported in the literature.9,10
The BZN powder was characterized by light scattering (Zeta-sizer 3000HS, Malvern Instruments, UK) for particle size, BET for surface area (Micrometrics, ASAP2000 BET, USA), elec-tronic probe microanalyzer (EPMA, Joel, JXA-8600SX, Japan) for chemical composition, and X-ray diffractometer (XRD, PW 1830, Philips, Nederland) for the crystalline structure. The BZN powder was pressed at 500 kg/cm2to form tablets (8 mm
diame-ter) and sintered at 1050◦C for 4 h. The distribution of elements in sintered BZN tablets was observed using an electronic probe microanalyzer. The microstructures of sintered samples were evaluated by scanning electron microscopy (SEM) equipped with wavelength dispersive spectrometer (WDS) (Joel, JSM-T100, Japan).
The dielectric properties of sintered BZN tablets were evalu-ated by Impedance Analyzer (Hewlett Packard, 4291B, USA) in the frequency range of 1 MHz–1.8 GHz and Network Analyzer (Hewlett Packard, 8722ES, USA) in the microwave frequency region. THz measurement (200–750 GHz) was performed using a custom-made time domain THz spectrometer with femtosec-ond laser system (Quantronix, Odin). Two identical 1 mm [1 1 0] ZnTe single crystals were used to generate (optical rectifica-tion) and detect (electro-optic sampling) the THz pulses. Infrared reflectivity spectra were obtained using a Fourier transform spec-trometer (Bruker IFS 113v, Germany) in the frequency range of 20–3300 cm−1(0.6–100 THz).
3. Results and discussion
The physical and chemical properties of Bi1.5Zn0.92
Nb1.5O6.92and Bi1.5ZnNb1.5O7powders prepared by the solid
mixing method are summarized inTable 1. They have compa-rable surface areas and particle size. The content of bismuth in both BZN powders was slightly reduced after calcination at 900◦C for 2 h. This might be due to the volatility of bismuth. Despite this, the stable cubic pyrochlore phase was preserved in both samples as shown in their XRD spectra (Fig. 1). Extra XRD peaks of ZnO are observed in the Bi1.5ZnNb1.5O7sample
as compared to Bi1.5Zn0.92Nb1.5O6.92. It corresponds to Levin et
al., who have shown that Bi1.5ZnNb1.5O7is thermodynamically
unstable, and Bi1.5ZnNb1.5O7consists of Bi1.5Zn0.92Nb1.5O6.92
and small amount of ZnO.12 For the structure and dielectric properties studies, both samples were sintered at 1050◦C for 4 h.
Table 1
Surface area, particle size, and chemical compositions of Bi1.5Zn0.92Nb1.5O6.92 and Bi1.5ZnNb1.5O7powders
Type of BZN Surface area (m2/g)
Particle size (m)
Chemical compositions after calcinations (mole)
Bi Zn Nb
Bi1.5Zn0.92Nb1.5O6.92 7.08 0.40 1.42 0.92 1.50 Bi1.5ZnNb1.5O7 7.01 0.43 1.43 1.00 1.50
Fig. 1. XRD patterns of Bi1.5Zn0.92Nb1.5O6.92and Bi1.5ZnNb1.5O7.
In order to obtain the distribution of elements in two samples, we used the EPMA technique to map the samples. The metal elements of Bi1.5Zn0.92Nb1.5O6.92are distributed uniformly in
the sample as shown inFig. 2. There is a slight Zn element congregation in the sample of Bi1.5ZnNb1.5O7as shown in the
upper right corner ofFig. 3.Table 2summaries the results of EPMA–WDS chemical composition analysis of two materials at different locations within the samples. It is interesting to note that in both the samples there are no differences in the grain or at its boundary. Both Bi1.5Zn0.92Nb1.5O6.92and Bi1.5ZnNb1.5O7
crystals exhibit many small voids (<1m) which is likely from the loss of volatile Bi during sintering (reduced from 1.43 to 1.42 mol).
In the structure of Bi1.5Zn0.92Nb1.5O6.92, 21% of Bi+3
atoms are replaced with Zn2+atoms, and 4% of the A position remains vacant (substitutional point defects) that provides room for dielectric relaxation.10 The temperature dependence of real and imaginary part on the dielectric function for Bi1.5Zn0.92Nb1.5O6.92between 3 MHz and 1.8 GHz is shown in
Fig. 4. The dielectric relaxation occurs between 100 and 230 K. The relaxation frequency is shifted to a higher temperature with increasing frequency. Similar behavior was described in Ref.11 The dielectric relaxation of Bi1.5Zn0.92Nb1.5O6.92 at
frequencies higher than 1.8 GHz could likely occur at room tem-perature. The presence of dielectric relaxation will result in high microwave dielectric loss. We have observed a low Q× f (less than 600) experimentally in the samples at 2.2 GHz (Table 3).
Experimental room temperature infrared reflectivity spec-trum of Bi1.5Zn0.92Nb1.5O6.92with its fit is shown inFig. 5(a).
The spectral range above 1000 cm−1is not shown because the reflectivity is flat at frequencies approaching the value given by the high-frequency permittivityε∞. The complex dielectric responseε*(ω) in the infrared range can be obtained from the reflectivity R (ω) spectra via formula13:
R(ω) = √ ε∗(ω) − 1 √ ε∗(ω) + 1 2 (1)
Fig. 2. EPMA element mapping of Bi1.5Zn0.92Nb1.5O6.92: (a) microstructure, (b) Bi element mapping, (c) Zn element mapping, and (d) Nb element mapping.
Table 2
Chemical compositions of sintered Bi1.5Zn0.92Nb1.5O6.92and Bi1.5ZnNb1.5O7 at different locations of the samples
Type of BZN Locating test at the sample
Chemical compositions after sintering (mole) Bi Zn Nb Bi1.5Zn0.92Nb1.5O6.92 In grain 1.43 0.92 1.50 At grain boundary 1.43 0.92 1.50 Bi1.5ZnNb1.5O7 In grain 1.42 1.00 1.50 At grain boundary 1.42 1.00 1.50
using the fit procedure of ε* (ω). A generalized oscillator model13with the factorized form of the complex dielectric func-tion was used:
ε∗(ω) = ε(ω) + iε(ω) = ε∞ n j=1 ω2 LOj− ω 2+ iωγ LOj ω2 TOj− ω2+ iωγTOj (2)
Fig. 4. Temperature dependence of dielectric permittivity at selected frequencies between 3 MHz and 1.8 GHz: (a) real part and (b) imaginary part of dielectric permittivity for Bi1.5Zn0.92Nb1.5O6.92.
Table 3
Dielectric properties of Bi1.5Zn0.92Nb1.5O6.92 and Bi1.5ZnNb1.5O7 at high frequency Composition f (GHz) ε Q× f (GHz) Sintering condition Bi1.5Zn0.92Nb1.5O6.92 2.27 121.3 487 1000◦C, 4 h 2.25 126.2 520 1050◦C, 4 h 2.22 130.7 368 1100◦C, 4 h Bi1.5ZnNb1.5O7 2.24 121.2 389 1000◦C, 4 h 2.24 126.0 426 1050◦C, 4 h 2.23 130.3 374 1100◦C, 4 h ω2 TOjandω 2
LOjdenote the transverse optical (TO) and
longitudi-nal optical (LO) frequencies of the jth polar mode, respectively, andγTOj andγLOj are the corresponding damping constants. Theε∞ denotes high-frequency permittivity originating from electron transitions. The results of the fittings are shown in
Fig. 5(a) by dotted lines.
Fig. 5. FTIR and THz spectra of Bi1.5Zn0.92Nb1.5O6.92: (a) infrared reflectivities at 300 K, (b) the real part of permittivity, and (c) the imaginary part of permittivity calculated from the fits to the reflectivities and submillimeter data with Eqs.(1) and (2).
The real part ε (ω) and imaginary part ε (ω) of com-plex permittivity obtained from the fit are shown inFig. 5(b) and (c) together with experimental THz spectra. The more accurate experimental THz dielectric data were used to fit the infrared reflectivity data of Bi1.5Zn0.92Nb1.5O6.92 in the range
of 6–30 cm−1.Fig. 5(c) also shows that the oscillator fit is not suitable for description of the THz data below 10 cm−1. The relaxation is probably broader and should be in fact fitted with the distribution of relaxation frequencies and not by overdamped oscillator how it is in our case. The submillimeter permittivityε is lower than the permittivity below 2 GHz which gives evidence about the presence of microwave relaxation. This broad relax-ation was also seen in Bi1.5ZnNb1.5O7and fitted in Ref.11with
distribution of relaxations. Levin et al.12have shown that the Bi
and Zn atoms in the A sites and some O atoms are dynamically disordered among several local potential minima, and Kamba et al.11 have assigned the relaxation to the local hopping of these disordered atoms. Ref.11is devoted to Bi1.5ZnNb1.5O7,
how-ever, later structural studies of Levin et al.12 revealed that the sample actually consisted of Bi1.5Zn0.92Nb1.5O6.92 and small
amounts of ZnO.
We note the broad absorption peak near 30 cm−1inε (ω) spectrum inFig. 5(c). It was fit to overdamped oscillator and gives evidence about large structural disorder in the BZN lat-tice. Parameters of the oscillator fit are similar to those already published in Refs.11,14. The results indicate that the polar phonon modes are not influenced by small content of ZnO second phase. The dielectric properties of Bi1.5Zn0.92Nb1.5O6.92 and
Bi1.5ZnNb1.5O7 at high frequencies are listed in Table 3.
Both materials reveal high dielectric constant and low Q× f. Their microwave properties are very similar even when chang-ing the sinterchang-ing temperatures. However, Bi1.5Zn0.92Nb1.5O6.92
exhibits a better Q× f than that of Bi1.5ZnNb1.5O7 especially
for the sample sintered at 1050◦C for 4 h. The excess ZnO in Bi1.5ZnNb1.5O7 may produce interstitial defects appearing in
the cubic pyrochlore crystalline structure. Such defects would reduce the Q× f of Bi1.5ZnNb1.5O7.
4. Conclusions
Ceramics of Bi1.5ZnNb1.5O7 and Bi1.5Zn0.92Nb1.5O6.92
were successfully prepared. Our microstructure study of Bi1.5ZnNb1.5O7 shows that it consists of cubic pyrochlore
structure of Bi1.5Zn0.92Nb1.5O6.92 and small amounts of ZnO.
For pure Bi1.5Zn0.92Nb1.5O6.92, a broad dielectric relaxation
that peaks around 230 K was observed at 1.8 GHz, which is similar to the relaxation of Bi1.5ZnNb1.5O7 seen in Ref.11.
Nevertheless, the Bi1.5Zn0.92Nb1.5O6.92 has systematically
slightly higher Q× f than that of Bi1.5ZnNb1.5O7. It is probably
due to interstitial defects from ZnO second phase presented in Bi1.5ZnNb1.5O7. Highly disordered pyrochlore structure results
not only in broad microwave dielectric relaxation but also in broad excitation in submillimetre range, which is probably
activated phonon density of states in FTIR and THz spectra. Both Bi1.5ZnNb1.5O7 and Bi1.5Zn0.92Nb1.5O6.92 reveal high
dielectric constant (>120) and low Q× f (<600) at 2.2 GHz. The low Q× f results from the dielectric relaxation and voids present in the samples.
Acknowledgments
Financial support obtained from the National Science Coun-cil of Republic of China (NSC91-2622-E-007-032, NSC92-2120-E-002-002, and NSC93-2120-M-002-010), and Academy of Sciences of the Czech Republic (A1010213) is highly appre-ciated. We are grateful to An-Jey Su of University of Pittsburgh for editing this manuscript and to A. Pashkin for doing the THz spectra measurements.
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